Optical recording media, such as large memory capacity optical discs, are known in the prior art to include non-erasable write-once systems which utilize as a recording layer a TeO.sub.x (0&lt;x&lt;2.0) thin film formed from Te and TeO.sub.2. Erasable discs are also known in which it is possible to repeatedly write and erase information by the application of optical energy. In such erasable optical discs, a thin film layer of recording material is heated and melted by laser light, then rapidly cooled so that its atomic structure is transformed into a substantially non-crystalline or amorphous state. Recorded information may be distinguished from the unrecorded regions of the optical disc by the optical properties of the amorphous state. The recorded information may subsequently be erased by again heating the thin film recording layer, and allowing it to cool so that its atomic structure is permitted to anneal This process transforms the heated regions of the recording layer into a substantially crystalline state which has optical properties which differ from that of the amorphous state to indicate thereby an erased condition. Reading of the recorded information is accomplished by utilizing a low power laser beam which does not create any structural changes in the recording layer.
Typically, laser light is focused to a spot of approximately 1 micron in diameter on the optical recording medium to heat a correspondingly small region of the thin film recording layer. Although heating occurs for only a short period of time, the recording layer is generally sandwiched between two heat resistant dielectric layers formed from SiO.sub.2 or other dielectric materials so as to avoid thermally damaging other layers of the recording medium. In some optical disc structures, the dielectric layer which covers the top of the thin film recording layer is itself covered by a reflecting layer, so that the laser beam may be reflected thereby. By incorporating a reflecting layer, the sensitivity to recording and erasing may be optimized by making use of optical interference effects. Since these optical interference effects depend upon the film thicknesses of the dielectric layers formed on each side of the thin film recording layer, it is very important that the film thicknesses of these layers be accurately controlled during manufacture.
Generally, the thin film recording layer, the two dielectric layers, and the reflecting layer are deposited on a substrate by a sputtering process which is capable of forming high quality films with very few defects. In the prior art, film thickness during sputter deposition has been monitored by using a quartz crystal oscillator positioned within the sputtering system so that it is simultaneously coated with the thin film being deposited. The resonant frequency of the quartz crystal oscillator is continuously altered by the mass of the coating to produce a signal indicative of the film thickness.
During use of such conventional quartz crystal thickness monitors, it is necessary to replace the quartz crystal oscillator at periodic intervals when the quartz crystal becomes coated with sufficient mass to exceed its operating limits. In a mass production environment, replacement of quartz crystal thickness monitors results in undesirable down time.
Optical monitoring of film thickness during deposition is also known in the prior art. During prior art optical monitoring, the surface upon which the film is depositing is irradiated with light, and a detector is appropriately positioned to monitor the intensity of either the reflected or transmitted light. As the film forms, the intensity of the reflected or transmitted light varies with film thickness as a result of changes in film transmissivity or interference effects, and the film deposition process is halted when a predetermined intensity variation has occurred. Since each layer must be individually monitored, prior art optical thickness measurements are not appropriate for mass production. Further, during optical monitoring, the optical detectors are often sensitive to noise produced by the electrical discharge in the sputtering system, and other systematic errors may also influence the accuracy of the film thickness measurement.